Industrial electrolysis systems in which brines of various kinds are subjected to electrolysis in order to produce other useful chemical products have been operating on a large scale for decades. In particular, chlor-alkali and chlorate electrolysis systems have been used to provide much of the chlorine, sodium hydroxide, and chlorate products which are subsequently used to prepare other chemicals or used in the manufacture of various other products.
As a consequence of increasing environmental concerns coupled with a highly competitive marketplace, modern chlor-alkali and chlorate producers are forced to look for alternative ways to minimize the amount of solid and liquid effluent produced as well as ways to reduce operating and capital costs.
A current strategy for reducing the amount of effluent is to use evaporated salt as a source of raw material to make-up the brine to be electrolyzed instead of the solar or rock salt typically used in the past. Evaporated salt is a much purer and cleaner source of salt and typically has amounts of alkali earth metal and other heavy metal contaminants that are orders of magnitude lower in concentration. Upon dissolution of this purer salt, the resulting brine solution quality is such that the conventional primary treatment process for the brine in such electrolysis systems can be eliminated.
In chlor-alkali systems, a supply of brine at an appropriate concentration is supplied to an electrolyzer where it is partially electrolyzed. Weak spent brine from the electrolyzer is then supplemented with additional make-up salt in a saturator and is then recycled back as brine supply for the electrolyzer. However, the conventional secondary treatment process for the brine in such systems uses a purification subsystem comprising cationic chelating resins, which are not effective in removing certain impurity species such as aluminum and silica. Historically, such impurity species were removed with the purge of sludges associated with the conventional primary treatment process. Thus, with the elimination of this primary treatment process, the aluminum and silica impurity species are not effectively removed by the secondary treatment process and consequently they can accumulate in the recycling brine circuit as make-up salt is continually added thereto.
These accumulating aluminum and silicon species impurities may be removed by continuously purging an amount of brine from the recycling brine in the main recirculation line in the system. The required amount of purge may vary from about 5 to 30% of the flow rate of the brine in the main recirculation line depending upon the purity of the supply of evaporated salt. However, the loss of salt associated with purging is generally not considered economical. Thus instead, such impurities are typically removed by treating the full flow of brine in the recirculation line.
Methods are disclosed in the art for removing aluminum and silicon species in brine streams. For instance, U.S. Pat. No. 4,073,706 teaches a process for the removal of trace metals from alkali halide brines. The addition of controlled amounts of magnesium ions to brine and subsequent precipitation of magnesium hydroxide removes metal contaminants, and provides a brine suitable for use in the electrolytic production of chlorine and alkali metal hydroxide. In the process, the pH can be adjusted by the addition of NaOH.
Also for example, U.S. Pat. No. 6,746,592 discloses a method for the reduction of soluble aluminum species in an evaporated salt alkali metal halide brine to provide a brine feedstock suitable for use in a chlor-alkali membrane cell process. The method comprising treating the brine with a suitable amount of magnesium salt and sufficient alkali metal hydroxide to provide an excess alkalinity concentration to effect precipitation of a magnesium aluminum hydroxide complex.
Further, U.S. Pat. No. 4,274,929 teaches a process for the removal of silicates in alkali metal choride containing industrial waste streams to provide waste brine streams suitable for use in the electrolytic production of chlorine and alkali using a diaphragm electrolytic cell. The process involves adding a soluble magnesium compound to alkali metal chloride solution and precipitating the silicates as compounds of magnesium. The process includes adjusting the pH of the alkali metal chloride solution to about 11.5 by adding sodium hydroxide, sodium carbonate, or mixtures thereof to render the magnesium silicates insoluble in the solution.
Although methods such as the above involving magnesium addition and precipitation are effective in removing aluminum and silicon species, the high levels required for silicon species removal result in poor filtration. And, much more expensive filteraid in the filtering subsystems must be used for reasonable filter cycle times. And while such methods have been used in the art to purify brine streams for or in chlor-alkali electrolysis systems, such methods do not appear to have been suggested for use in side streams in such systems. Instead, the methods are used or suggested for use in a main line or recirculation line for the brine streams. Further, such methods do not seem employed in chlorate electrolysis systems.
Despite the maturity and sophistication of modern electrolysis systems, there remains a continuing need for reduction of effluent and for reduction in operating and capital costs. The present invention addresses these and other needs as discussed below.